1. Field of the Invention
The present invention relates to a thyristor having a switchable emitter short circuit, such as of claim 1. Such a is known, for example, from German Offenlegungsschrift No. 3,118,347.
2. Discussion of Background
In thyristor technology, local fixed emitter short circuits have been installed for a long time on the cathode side of the thyristor in order to ensure that the component is capable of turning off even with a very rapid rise (dU.sub.A /dt) of the anode voltage (see, for example, Gentry, F. E.; et al. "Semiconductor Controlled Rectifiers", Prentice Hall Inc., 1964, page 138 f.). Due to the emitter short circuits, displacement currents such as occur during rapid voltage rises can flow away without gating the npn part-transistor of the thyristor.
Besides the advantage of improved turn-off capability, however, the fixed emitter short circuits have the disadvantage of decreased triggering sensitivity and of an increased holding current. In addition, the short circuits represent an obstruction to the propagation of the triggering front which has the effect of limiting the rate of current rise (dI/dt) on turn-on.
To avoid these disadvantages, it has been proposed (see, for example, German Offenlegungsschrift No. 3,118,353), to switch the emitter short circuits on and off in accordance with the switching state of the thyristor, the emitter short circuits being interrupted or disconnected during the firing process and in the conducting state of the thyristor and being connected during the turn-off state.
In the printed document mentioned, the switchability of the emitter short circuits is achieved by the fact that the regions of the thyristor p-base layer appearing at the surface are contacted independently of the cathode and can be conductively connected to the cathode via an external field effect transistor (FET) acting as a switch.
According to the teaching from the initially mentioned German Offenlegungsschrift No. 3,118,347, the FET switch can also be monolithically integrated directly into the thyristor structure in the form of an MIS-FET structure.
These known solutions have the common feature that field effect transistors are used as switching elements for the emitter short circuits. Since, in principle, the driving voltage for the current flowing in the emitter short circuit is less than about 0.7 Volt, the aim in dimensioning must be as low as possible a path resistance in order to achieve a sufficiently high short-circuit current. This requires small lateral distances between the individual short circuits and low resistances in the switching transistors in the switched-in state.
Since the field effect transistors, however, display an approximately resistive behavior with respect to their conducting-state characteristic, that is to say the voltage dropped across the switch always increases approximately proportionally to the current, the maximum short-circuit current is limited right from the start by the structure of the switching element.